Crumb Trail
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Wednesday, February 15, 2006

Faster evolution.

During photosynthesis, plants and some bacteria convert sunlight and carbon dioxide into usable chemical energy. Scientists have long known that this process relies on the enzyme rubulose 1,5-bisphosphate carboxylase/oxygenase, also called RuBisCO. While RuBisCO is the most abundant enzyme in the world, it is also one of the least efficient. As Dr. Matsu-mura says, "All life pretty much depends on the function on this enzyme. It actually has had billions of years to improve, but remains about a thousand times slower than most other enzymes. Plants have to make tons of it just to stay alive." . .

For decades, scientists have struggled to engineer a variant of the enzyme that would more quickly convert carbon dioxide. Their attempts primarily focused on mutating specific amino acids within RuBisCO, and then seeing if the change affected carbon dioxide conver-sion. Because of RuBisCO's structural complexity, the mutations did not have the desired outcome.

For their own study, Dr. Matsumura and his colleagues decided to use a process called "di-rected evolution" which involved isolating and randomly mutating genes, and then inserting the mutated genes into bacteria (in this case Escherichia coli, or E. coli). They then screened the resulting mutant proteins for the fastest and most efficient enzymes. "We decided to do what nature does, but at a much faster pace." Dr. Matsumura says. "Essentially we're using evolu-tion as a tool to engineer the protein."

Because E. coli does not normally participate in photosynthesis or carbon dioxide conversion, it does not usually carry the RuBisCO enzyme. In this study, Matsumura's team added the genes encoding RuBisCO and a helper enzyme to E. coli, enabling it to change carbon dioxide into con-sumable energy. The scientists withheld other nutrients from this genetically modified organism so that it would need RuBisCO and carbon dioxide to survive under these stringent conditions.

They then randomly mutated the RuBisCO gene, and added these mutant genes to the modified E. coli. The fastest growing strains carried mutated RuBisCO genes that produced a larger quantity of the enzyme, leading to faster assimilation of carbon dioxide gas. "These mutations caused a 500 percent increase in RuBisCO expression" Dr. Matsumura says.

The spin is that plants using more CO2 will affect climate, but this seems silly since there are always other limits to growth. More realistically, it may help selected crops grow and yield.

posted by back40 | 2/15/2006 09:31:00 AM


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